The present invention relates generally the design of gas turbine engine containment casings. In particular it relates to the design of containment casings which are provided to contain a fan blade, within the engine, during failure of a fan blade.
Turbo fan gas turbine engines are commonly provided with a fan rotor in the forward portion of the engine. The rotor assembly includes a rotor disc and a number of fan rotor blades. The fan rotor blades extend outwardly from the disc across a flow path for the working medium gases. These blades are more cambered and are considerably larger than the blades of the axial flow core compressors and turbines used in such engines.
The fan rotor blades are driven at high rotational speeds about an axis of rotation to provide the first stage of compression to the working medium gases, and propulsive thrust. Foreign objects, such as birds, hailstones or other objects, which are on occasion ingested into the engine with the working medium gases may strike and damage the blade to such an extent that the blade fails in the root region of the fan blade where it is attached to the rotor disc. The blade may also fail in the root region due to other causes. During such a failure a rotor blade may be hurled outwardly from the rotor assembly with considerable energy and at high speed.
Because of the size of the fan blades and the speeds at which they may be released in the event of failure, fan blades present a difficult containment problem. It being appreciated that it is desirable to contain a failed fan blade within the engine so that further damage to the aircraft or surrounding objects does not occur.
One known design of containment casing is described in U.S. Pat. No. 4,417,848. According to this patent an annular containment casing surrounds the outer portions of the fan blades. The thickness of this casing increases in an axially rearward direction such that the casing has a maximum thickness rearward of a plane passing through the mid-chord point of the rotor blades. This increase in thickness selectively reinforces the case against the predicted impact of blade fragments such that the blade does not penetrate the casing and is contained.
A problem with this design is that in order to withstand large fan blades and/or high impact velocities the casing thickness needs to be considerable and over a large area of the casing. This undesirably adds considerable weight to the engine casing. Furthermore such a containment casing design is not optimal and can be further improved.
It is therefore desirable to provide a containment casing that addresses the aforementioned problem and/or provides improvements to containment casings generally.